This invention relates to electromechanical assemblies of packaged integrated circuits; and more particularly, it relates to such assemblies which: (1) allow defective integrated circuit die to be easily repaired, (2) allow for the assembly to be fabricated at low cost, and (3) allow signal transmission through the assembly at high speeds.
For comparison purposes, FIG. 1 illustrates a common (and perhaps the most widely used) integrated circuit package 10 of the prior art. Package 10 includes several square-shaped thin flat ceramic layers 11 that are laminated together. The bottom ceramic layer is solid, while the other layers have a central hole 12. A silicon integrated circuit die 13 is disposed in hole 12 and attached to the bottom ceramic layer. Ceramic and silicon have similar thermal expansion coefficients, and thus the die stays attached to the ceramic when package 10 is thermally cycled.
Bonding wires 14 carry signals between die 13 and conductors 15, the latter of which are screened on the internal ceramic layers 11. These conductors 15 make contact to a plurality of stiff input/output pins 16 which are attached to the top ceramic layer. To complete package 10, a heat sink (not shown) is attached to the bottom ceramic layer, and a lid (not shown) is attached to the top ceramic layer over die 13. Thereafter, a system which includes multiple integrated circuits 13 is made by soldering the pins 6 of several packages 10 into a printed circuit board.
One problem, however, with the above-described integrated circuit package 10 is that it is difficult to desolder from a printed circuit board. This problem occurs because the package has so many input/output pins. In FIG. 1, the package has 140 pins; and the trend in the semiconductor industry is for the number of pins to increase. With a large number of pins, heating the solder around each pin individually with a soldering iron and removing the melted solder individually with a solder sucker simply takes too long. Also, any removal process heats the printed circuit board in the area near the pins. This often causes conductors in that area of the board to delaminate or causes insulation resistance failures in the board.
Another problem with the prior art package 10 and systems which are made of the same is that they are inherently expensive. Some of the expense items are the cost of the input/output pins 16, the cost of the ceramic layers 11, and the cost of the heat sink. Another expense item is the high cost of equipment that is required to fabricate the package. This equipment includes a press and a high temperature furnace which are required to respectively laminate and cure the ceramic 11.
Still another problem with package 10 is the limitation that it places on the speed at which electrical signals travel between two die 13 in a system. In part, this limitation is due to capacitive coupling between the conductors 15 and the surrounding ceramic 11. This coupling causes a time delay Td for signals traveling from the pins 15 to the die 13 which equals 1.016 (ER).sup.1/2 nanoseconds per foot, where ER is the dielectric constant of the ceramic. In addition, package 10 is inherently larger than die 13. Thus, package 10 increases the distance which signals between two die in a system must travel, and that further limits the system's speed.
Accordingly, a primary object of the invention is to provide an improved assembly of intercoupled integrated circuit die.
Another object of the invention is to provide an assembly of intercoupled integrated circuit die which is easy to repair, low in cost, and high in speed of operation.